There are known security lights, commonly used for security areas such as parking lots, which use a conventional high pressure sodium (HPS) lamp or mercury vapor (MV) lamp. The HPS and MV lamps are normally operated continuously at full brightness, except when they are off. They have traditionally been preferred for security applications because they have significantly longer operational lifetimes than incandescent lamps, for example on the order of 15,000 to 20,000 hours in comparison to about 2,000 hours for an incandescent lamp. The longer operational lifetime is important not only to optimize the security effect of the bulb, but also because of the inconvenience involved in frequently changing incandescent lights which may be 20 feet in the air on a light pole in a parking lot. While these known lights have been generally adequate for their intended purposes, they have not been satisfactory in all respects.
More specifically, MV lamps are not the most energy efficient lamps available, and may be made illegal in the near future for most or all applications. Further, it is not considered appropriate to operate MV or HPS lamps under control of a sensor so that they are normally off but are turned on in response to detection of a condition such as motion, both because HPS and MV lamps usually take several minutes to progressively turn on, and because turning them on and off degrades them and should therefore be done only once a day in order to optimize their operational lifetimes.
There are other existing security lights which have the capability to turn on rapidly. These lights typically have an incandescent lamp, a passive infrared detector, a photocell, and a circuit which keeps the lamp off when the photocell is detecting daylight, which keeps the lamp off at night in the absence of detection of motion by the infrared detector, and which turns the lamp on for a predetermined time interval in response to detection of motion at night. These known units also have an override mode, which a user can actuate by turning power to the unit off and on with a wall switch that supplies electrical power to the light. In the override mode, the light is forced continuously on until the user disables the override mode using the wall switch, or until the photocell detects daylight and disables the override mode.
This latter type of security light is typically not used in applications such as a commercial or industrial parking lot where light must be provided continuously at night, because of the power consumption of the incandescent bulbs, and because of the somewhat limited life of incandescent bulbs when operated at full brightness. The lifetime of a typical incandescent bulb may be only about 2000 hours. In addition, these known security lights are either fully off or fully on, and are thus not suitable for use in certain security applications where the light must be emitting illumination in order to be useful.
An object of the present invention is therefore to provide a light which has a lamp with a relatively long operational lifetime, and which can provide illumination throughout the night in a manner which is highly energy efficient and is suitable for security applications.